国家天文台“极端”自适应光学仪器在智利完成客座观测

[Dr. Claudio Ricci, a CAS-CONICYT Postdoctoral Fellow in astronomy, led an important paper in《Nature》on September 27th 2017, in which he reported recent major progress regarding how radiation feedback controls the shape of close environment around super-massive black holes, drawn from a multi-band survey of a sample of black holes selected in the hard X-ray band.]

A black hole is a place in space-time where gravity pulls are so strong that even light cannot get out, it is therefore “black” in any bands. The theory of general relativity predicts that a sufficiently large and dense mass would deform space -time and give birth to a black hole. Despite its invisible interior, a black hole can be indirectly inferred and investigated at various wavelengths through its interactions with the surrounding and in-falling materials.

It is known for decades that very heavy black holes inhabit the centers of galaxies (including our own galaxy, the Milky Way), but are hidden by gas and dust. Some of these black holes can “eat” materials from their environment, and emit a lot of light during this process. Most of these “luminous” black holes are surrounded by large amount of gas and dust, distributed in a doughnut-like structure. Such a structure could resemble a pantry, which guarantees that the black hole can keep eating, radiating and growing. However, it is not known where exactly this material is located, and what the relationship is between light produced by the black hole and the dusty gas.

In order to address this long-standing issue, Dr. Claudio Ricci, a postdoc fellow supported by Chinese Academy of Sciences South America Center for Astronomy (CASSACA), and his collaborators made use of observations carried out in the X-ray band, similar to what is typically used for radiographies in hospitals. With each observation performing these “space radiographies”, they could measure the amount of material around the black hole, and then study its evolution.

This project started in 2013, and it took the authors many years to create the large database used for their research, using data from space telescopes as well as ground-based observatories, such as those in Chile. The Chilean telescopes were extremely important for measuring the properties of the black holes, and in particular, for “weighing” their masses. The main X-ray instrument used was the NASA satellite Swift, but also data from the satellites XMM-Newton of ESA, Suzaku of JAXA, and another NASA telescope, Chandra, were used. In the optical band the facilities used include the Sloan Digital Sky Survey, the UK Schmidt telescope, Gemini, CTIO, DuPont and SAAO.

With this work, Ricci and collaborators discovered the process that controls the interaction between light produced by the black hole and the gas that surrounds it, and showed that most of the material around black holes is located close to it. The authors found that, when the black hole emits a lot of light, this light pushes away the material from its vicinity; in other words, the gas can “evaporate” because of the large amount of energy released by the material falling rapidly onto the black hole. This could also mean that, if the black hole “eats” too rapidly, the energy produced could destroy the “food” available for the future.

It is a major step forward to reveal a clear picture of the connection between radiation feedback and the surrounding material’s shape. “The next step will be to further understand the details of this behavior, and what happens to the material that is pushed away from the black hole”, said Dr. Ricci, the leading author of this work.

Figure 1. Artistic impression of the gas and dust surrounding an accreting supermassive black hole. Taken from NASA/JPL/Caltech.

Figure 2. Schematic representation of the material surrounding supermassive black holes for different ranges of Eddington ratio. The Eddington ratio is the ratio between the bolometric and the Eddington luminosity, where the latter is defined as the luminosity at which the radiation pressure from a source, in this case the accreting SMBH, balances the gravitational attraction. Taken from Ricci et al. (2017, Nature Letter).

Dr. Claudio Ricci, a CAS-CONICYT Postdoctoral Fellow in astronomy, recently authored a review paper in 《Nature Astronomy》 with Cristina Ramos Almeida of IAC Tenerife, in which they summarized the most important developments of the past ten years in the fields of accreting black holes and circum-nuclear materials, as revealed by observations in the X-ray and infrared bands.

All massive galaxies host supermassive black holes (SMBH) at their centers, and these objects are often found to be hidden behind large amounts of gas and dust. This circum-nuclear material is what eventually accretes onto the black hole, allowing it to grow, and its structure and evolution have been the subject of intense study in the past decade. Chinese Academy of Sciences South America Center for Astronomy (CASSACA)’s postdoctoral fellow Claudio Ricci and Dr. Cristina Ramos Almeida (IAC Tenerife) were recently invited to write a review for《Nature Astronomy》 on this subject, with the idea of combining results obtained from X-ray and infrared studies of the close environments of supermassive black holes. These two energy bands are highly complementary: while X-rays are produced very close to the supermassive black hole and allow the study of radiation absorbed and reflected, infrared radiation is directly produced by the dust around the black hole.

A black hole is a place in space where gravity pulls so much that even light cannot get out, and therefore itself is “black” in any bands. The theory of general relativity predicts that a sufficiently large and compact mass can deform space time and give birth to a black hole. Black holes of stellar masses are expected to form when very massive stars collapse at the end of their life cycle. After a black hole is formed, it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may form. The first widely-accepted black hole is Cygnus X-1 discovered in 1964, and it weighs about 15 solar masses (Figure 1). The well-known radio source named Sagittarius A*, sitting at the core of our own Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses.

Despite its invisible interior, the presence of a black hole can be inferred through its interactions with matter. Circum-nuclear materials falls onto a black hole, and forms an AGN (active galactic nucleus), one of the brightest objects in the universe, including an external accretion disk, an X-ray emitting corona, a broad-line region (BLR), a torus and a narrow-line region (NLR) (Figure 2), which have various physical characteristics, and their radiation are widely used to study the black hole and its surroundings.

The review summarizes the most important developments of the past ten years in this field, and particularly in light of the most advanced X-ray and IR facilities. The authors discuss why the circum-nuclear material is anisotropic, clumpy, and connects with the host galaxy via gas inflow/outflows. They also highlight the importance of dust emission from the polar region of the AGN, possibly related to outflows caused by radiation pressure from the accreting supermassive black hole. Future observing facilities will allow a better understanding of the nuclear environment of AGN, and a fuller description on how it links the black hole to its host galaxy. In particular, the upcoming James Webb Space Telescope (JWST) will allow us to probe the structure and evolution of AGN’s polar dust, while high-resolution spectrometers, such as the ones on board XARM and Athena satellites, will shed light on the physical characteristics of the gas and dust.

Dr. Ricci was invited to write this review because of his significant contribution to this field of research, and in particular in the light of his recent studies on the properties of the most heavily obscured accretion events in the Universe, and of the discovery that the merger of two or more galaxies can strongly affect the surroundings of the supermassive black hole, enriching it in gas and dust. Being able to publish review papers in such prestigious scholarly journals is usually an indication of wide acceptance and recognition of the author’s research work.

For young scientists such as Ricci and Almeida, this is an even more remarkable testimonial to their creative contribution to the relevant subject matter.

Figure 1. An artist’s drawing of a black hole named Cygnus X-1. It formed when a large star caved in. This black hole pulls matter from the blue star beside it. Taken from NASA/CXC/M. Weiss.

Figure 2. Sketch of the main AGN structures, seen along the equatorial and polar directions. From the center to host-galaxy scales: SMBH (Super Massive Black Hole), accretion disk and X-ray emitting corona, BLR (Broad-Line Region), torus and NLR (Narrow-Line Region). Different colors indicate different compositions or densities. Taken from Ramos Almeida & Ricci (2017, Nature Astronomy).

For more information about the Nature Astronomy Review, please visit https://www.nature.com/articles/s41550-017-0232-z.

Ministry of Science and Technology of China announces the 2017 China-Latin-American Young Scientist Exchange Program

To promote the long-term scientific and technological partnership between China and Latin American and Caribbean (LAC) countries, the Ministry of Science and Technology of China (MOST) decides to continue the “China-Latin America Young Scholars Exchange Program” for all institutes, universities and enterprises this year in LAC. Interested researchers are encouraged to submit applications.

Specific requirements are as follows:

Job requirements

Not classified

Not diplomatic education;

Scientific research positions with clear plans and responsibilities;

The young scientists must work with Chinese research team;

Working in China for 6 months or 12 months, and can be extended once when necessary

Appropriate applicants

Young scientists from Latin America and the Caribbean countries, with preferences to those countries with bilateral intergovernmental committees, subcommittees for the cooperation in science and technology (like Cuba, Mexico, Brazil, Chile, Argentina, Uruguay, Colombia, Ecuador, Costa Rica, etc.)

Under the age of 45;

With more than 5 years of scientific research experience, or a doctorate degree;

Good at English or Chinese;

Physical and mental healthy, and capable of conducting scientific research work;

Commitment to full-time work in China during the grant period;

Commitment to comply with relevant Chinese laws during China.

Funding methods and standards

MOST will provide 12, 500 yuan per person per month for young scientists included in the Program, to cover their housing subsidy, living allowance and various types of insurance in China. In principle, international travel expenses shall be excluded, to ensure the smooth implementation of the project. However, if buying international air tickets is indeed necessary for the awarded young scientists, the cost will be subtracted from their living allowance;

The funds will be allocated to the Chinese institutions, which will host the awarded young scientist through the China Science and Technology Exchange Center. All the funds shall be only used for the work and living of the young Chinese scientists staying in China, and the host institution shall not deduct or accrue the management fees.

Application and funding procedures

Fill in the 2017 China-LAC Young Scientists Exchange Program Application Form and the recommendation form. The latter form must be stamped and signed by the sending institution of the applicant. Please send the two forms (pdf files) to cassaca@bao.ac.cn.

The China Science and Technology Exchange Center (CSTEC) will review and select of the applications, and submits it to the International Cooperation Department (ICD) of the MOST for final decision, according to the emphasis of the intergovernmental cooperation in science and technology. For the awarded application, project to confirm the funding, CSTEC will inform the Chinese host institutions in written form, and sign the contract with them

The host institution and group should sign a working agreement with the LAC young scientists, to clarify the responsibilities, obligations, and describe the proposed collaboration research programs.

The host institution and group is responsible for assisting the LAC young scientists to handle all necessary paperwork required for visa applications, and help to arrange accommodations in China.

The deadline for the application is September 15, 2017. If there is anything unclear, please feel free to contact Dr. Wei Wang at wangw@bao.ac.cn.

This illustration compares growing supermassive black holes in two different kinds of galaxies. A growing supermassive black hole in a normal galaxy would have a donut-shaped structure of gas and dust around it (left). In a merging galaxy, a sphere of material obscures the black hole (right).

Credits: National Astronomical Observatory of Japan

Black holes get a bad rap in popular culture for swallowing everything in their environments. In reality, stars, gas and dust can orbit black holes for long periods of time, until a major disruption pushes the material in.

A merger of two galaxies is one such disruption. As the galaxies combine and their central black holes approach each other, gas and dust in the vicinity are pushed onto their respective black holes. An enormous amount of high-energy radiation is released as material spirals rapidly toward the hungry black hole, which becomes what astronomers call an active galactic nucleus (AGN).

A study using NASA’s NuSTAR telescope shows that in the late stages of galaxy mergers, so much gas and dust falls toward a black hole that the extremely bright AGN is enshrouded. The combined effect of the gravity of the two galaxies slows the rotational speeds of gas and dust that would otherwise be orbiting freely. This loss of energy makes the material fall onto the black hole.

“The further along the merger is, the more enshrouded the AGN will be,” said Claudio Ricci, lead author of the study published in the Monthly Notices Royal Astronomical Society. “Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust.”

Ricci and colleagues observed the penetrating high-energy X-ray emission from 52 galaxies. About half of them were in the later stages of merging. Because NuSTAR is very sensitive to detecting the highest-energy X-rays, it was critical in establishing how much light escapes the sphere of gas and dust covering an AGN.

The study was published in the Monthly Notices of the Royal Astronomical Society. Researchers compared NuSTAR observations of the galaxies with data from NASA’s Swift and Chandra and ESA’s XMM-Newton observatories, which look at lower energy components of the X-ray spectrum. If high-energy X-rays are detected from a galaxy, but low-energy X-rays are not, that is a sign that an AGN is heavily obscured.

The study helps confirm the longstanding idea that an AGN’s black hole does most of its eating while enshrouded during the late stages of a merger.

“A supermassive black hole grows rapidly during these mergers,” Ricci said. “The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy.”

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR’s mission operations center is at UC Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror archive. JPL is managed by Caltech for NASA.

On November 22nd, in Santiago, the capital of Chile, at the witness of Chinese President Xi JinPing and Chilean President Michelle Bachelet, Prof. Jun Yan, Director General of National Astronomical Observatories, Chinese Academy of Sciences (NAOC), and Mr. Jorge Tabilo Álvarez, President of the Catholic University of the North (UCN) signed a Memorandum of Understanding (MOU) to jointly develop an Astronomical Observational base in Chile.

Due to its outstanding astronomical climate, and the steady support of its government, Chile is a key site for international astronomy research. Over the last few decades, several foreign interests, such as the U.S.A., Japan, and the E.U., have developed observational facilities in the country. The Ventarrones site (VTRS), which belongs to the Catholic University of the North, has been subject to detailed investigation and comparison by the Chinese Academy of Sciences South America Center for Astronomy (CASSACA) and is considered as an excellent site for astronomic observations. The site is located in the famous Atacama Desert, about 1200 KMs north from Santiago, and is at an altitude of 2900 meters.

The VTRS site was previously selected as a candidate site for the European Southern Observatory’s (ESO) 39 meter European Extremely Large Telescope (E-ELT, in construction), and its sky quality and conditions have been evaluated and positively confirmed by world-class institutions. In order to promote scientific developments in the field of observational astronomy and related sciences and technology, and to strengthen the collaboration and communication of scientists, engineers, and students between the two countries, NAOC and UCN have agreed to form a bilateral partnership to develop the VTRS site into a world-class scientific research base for observational astronomy. The site is also expected to play an important role in the scientific outreach and education.

This event marks the second time these parties have signed a MOU witnessed by both their respective heads of state, the first being the attendance of Chinese Premier Li Keqiang and the Chilean President in the signing ceremony of China-Chile Joint Astronomical Data Center in 2015. The MOU is also a highlight of and a major milestone in CASSACA’s past three years of work, after its foundation in Chile since 2013. Dr. Jun Yan，the Director General of NAOC, specifically pointed out that developing an observational base in Chile is a strategic step that has been carefully considered by the scientific community, based on the long-term plan for China’s development in the field of astronomy. The official signing of the MOU and its gradual implementation will enable China for the first time to have a world-class site overseas by means of joint development. This will be of great significance to the development of astronomical observation and techniques in China, and it will further promote cooperation and communications between China and Chile in the field.

The news that NAOC and UCN will collaborate to develop astronomical observatories in Chile has attracted strong interest in the local community and media. The idea of constructing future observatories in this kind of collaborative way is supported by the Chilean government and local astronomers, who are optimistic in regards to the future development of the project. One major media outlet in Santiago prepared a special report on the signing of the agreement, pointing out that this is a milestone in China’s development of astronomical observations in South America. Dr. Cameron, the director of ESO (Chile), commented in a congratulatory letter that this collaborative approach is an “excellent choice” for China. He also expressed willingness to provide assistance for the future development of the VTRS site.